Urban air pollution. Secondhand smoke. Slow suffocation. These are terms typically associated with carbon monoxide. Yet a new study finds that northern elephant seals (Mirounga angustirostris, pictured) love the stuff. Researchers discovered exorbitant blood levels of the gas in seals that rival those in people who smoke 40 cigarettes per day. The tubby mammals aren’t sneaking Marlboros while on shore, but merely protecting their organs from the consequences of deep-sea diving, according to a study published online today in The Journal of Experimental Biology. When hunting at sea, elephant seals take a deep breath at the surface and then plunge to depths between 300 and 800 meters. To conserve oxygen, the animals shut down blood flow to less critical organs, like the kidneys and liver. Cutting blood supply to a body region—known as ischemia—is usually harmful in animals. On the flip side, sudden restoration of oxygen flow—known as reperfusion—can damage organs, too. Research over the last decade, however, has revealed that low levels of carbon monoxide ward off internal injuries caused by such stress, and the team argues that’s why elephant seals produce large amounts of the gas.